Secretary of State Marco Rubio said the United States was prepared to enter “into very serious talks” about Iran’s nuclear program if Iran reopened the Strait of Hormuz, signaling that the Trump administration was prepared to accept an interim agreement that didn’t immediately take away Iran’s ability to make nuclear weapons.

“You can’t do a nuclear thing in 72 hours on the back of a napkin,” Mr. Rubio said in a brief interview Sunday during his visit to New Delhi. “The straits have to be immediately reopened, and then we will enter, under agreed-to parameters, into very serious talks about enrichment, about the highly enriched uranium and about their pledge to never have nuclear weapons.”

Neither the United States nor Iran has released details publicly, but on Sunday a U.S. official said there was an agreement in principle to a deal that would reopen the Strait of Hormuz with a commitment from Iran to dispose of their highly enriched uranium. Iran has not commented publicly, nor confirmed its stance on its stockpile of enriched uranium.

Mr. Rubio insisted that Mr. Trump’s policy that Iran could not have a nuclear weapon had not changed. But he signaled that the United States was prepared to adopt a phased approach to the talks, one that critics have said could take away Mr. Trump’s leverage in the negotiations.

“It can’t take years, but it’ll take some time to work through those technical matters,” Mr. Rubio said.

Referring to Mr. Trump, he added: “He’s always going to prefer to address it diplomatically, through agreements, and that’s why we’re going to exhaust every opportunity to achieve one. So right now, we have seven or eight countries in the region that are endorsing this approach, and we’re prepared to move forward on this approach.”

Mr. Rubio suggested that the United States could renew its threats to attack Iran if the negotiations do not bear fruit within two months. “Ultimately, the approach has to deliver what we want it to deliver,” Mr. Rubio said. “If it doesn’t, then the president has every option available to him in 60 days that he has available to him now.”